Electronic watches typically comprise quartz oscillators for generating timing signals. Quartz electronic watches are popular because they are reasonably accurate at reasonably low costs. Among the various types of quartz electronic watches, analogue-type electronic watches comprising hour, minute and/or second arms resembling that of a traditional watch are an important variety.
Electronic watches, whether digital or analogue, are typically built around a quartz oscillator circuitry which generates a timing oscillation. This timing oscillation produces a fundamental timing frequency in the form of pulses which are also commonly referred to as “clock signals”. An exemplary timing oscillation suitable for electronic watch applications is at 32.768 kHz. In case of an analogue electronic watch, the timing oscillation is converted into pulses for driving a stepper motor by a stepper motor driver circuit. The stepper motor in turn drives the time indicating arms. Typically, the motor driving pulses have a period of either 1 second or 60 seconds so that the second or the minute arm will move by 1 graduation at the intervals of a second or a minute respectively. The oscillator circuitry and the stepper motor driver circuit are usually integrated on a single integrated circuit (IC) chip for a compact, slim design and/or for costs saving.
FIG. 1 shows an exemplary quartz oscillator circuit which is more commonly known as the CMOS Pierce oscillator circuit. The Pierce oscillator circuit is widely used for timing signal generation in a digital electronic timing circuit and includes quartz crystal operating in the parallel mode.
To maintain a reasonable timing accuracy for timing integrity, the oscillation frequency of an oscillator circuit should be sufficiently accurate. For example, in a digital watch, an error of 20 ppm (parts per million) will result in a timing deviation of approximately 1 minute per month. In general, the accuracy of a crystal-based oscillator circuit depends mainly on the accuracy of the crystal and the capacitive load. For example, the oscillation frequency will increase if the capacitive load is decreased. Conversely, the oscillation frequency will decrease with an increase in the capacitive load.
Quartz crystals are commercially available as discrete components with a specified typical characteristic oscillation frequency when connected with a parallel capacitive load of a predetermined specific load capacitance. For electronic watch applications, a discrete quartz crystal is typically connected to an integrated watch circuit for forming a timing oscillator.
In an exemplary equivalent circuit of a typical oscillator shown in FIG. 2, the components CO, C1, L1 and R1, are the typical characteristic inherent parameters of a quartz crystal. The ratio of CO to C1, is a parameter representative of the inter-conversion between electrical and mechanical energy stored in the crystal. The component CL is equivalent to the two on-chip load capacitance measured in series. To generate an accurate oscillator operating frequency, the crystal parameters CO, C1, L1, R1 and the load capacitance CL have to be properly matched, for example, by on-chip fine-tuning capacitors.
However, on-chip capacitors for digital watch applications usually have a manufacturing tolerance of approximately 10% to 20%. To alleviate timing inaccuracy due to such tolerance in a typical digital watch production line, quartz crystals and silicon integrated chips are usually sorted individually for optimal matching. Such a process is typically manual, slow and expensive. Furthermore, for quartz crystals and silicon chips that cannot be matched, they have to be thrown away, thereby adversely affecting the overall production yield and increasing the overall production costs. On the other hand, it is well-known that on-chip capacitors and quartz crystals are very expensive and difficult to manufacture with very high precision.
Hence, it will be desirable if integrated circuit-chips for watch applications comprising means to fine-tune the oscillation frequency of the quartz oscillator circuitry can be provided in order to enhance timing accuracy without requiring unduly complicated or expensive circuitry or designs. Furthermore, it will be appreciated that external connection pins or pads on an integrated circuit chip are a valuable resource since additional pins or pads beyond an optimum number will mean increased package and production costs. Hence, it will be desirable if multiple-step fine-tuning can be achieved with minimum number of interfacing pad/pin. This is even better if no additional external logic circuitry and pinning requirements and/or with pinning configuration is required so that the integrated circuit is compatible with conventional watch IC layout for direct replacement so that manufacturing costs can be optimised and costs for PCB or tooling re-design can also be saved.